While oncogenic viruses are the initiators of a complex chain of events leading to malignancy, the host cell also exerts control over the expression of transformation. The elucidation of the host cell factors which are important in the expression of transformation by polyoma and SV40 and the problem of how they interact with viral functions represent the objective of this research. We are planning to study the factors which determine the stable association between the viral genome and the cellular genome in polyoma transformed cells, mainly by taking advantage of the situation found in polyoma tranformed rat cells, where the viral DNA is generally present both in an integrated and non-integrated state. We will also study the regulation of the synthesis of T-antigen and viral m-RNA in SV40-tranformed cells. Expression of these viral functions appears to be controlled by the position of the transformed cell in the cell cycle.